Engines may utilize EGR to reduce emissions and increase efficiency. For example, diesel engines may use EGR to address NOx generation during combustion. When the engine operates with boosting, such as via a turbocharger, the engine output may be limited based on the temperature of intake air. As such, hot EGR may reduce available engine output, as it heats the incoming fresh charge.
As such, in some systems, an EGR cooler, or a plurality of EGR coolers, may be used. For example, a plurality of EGR coolers in series may be used, where the coolers are configured differently to provide different amounts of cooling. Such a system is described in WO2005095780.
However, the inventors herein have recognized several issues with the above approach, and other approaches. In particular, while such configurations may enable multi-stage cooling, they may also lead to cooler fouling (overcooling and condensation) under some conditions. While such fouling may be avoided by reducing EGR flow, this then may result in increased emissions.
The above issues may be addressed, at least partially, by a system that coordinates EGR flow through and around a plurality of coolers differently depending on operating conditions. Thus, the inventors herein have recognized that, depending on operating conditions, EGR flow may be modulated through separate/different pathways to achieve an intake temperature (and/or EGR flow, EGR flow temperature, etc.) while also avoid individual cooler fouling conditions.
Further, the inventors herein have recognized an approach to address a constraint often experienced in EGR system design. In particular, EGR coolers may be designed for the high cooling load point. Then, under lighter loads, because over-cooling may occur, EGR bypass may be used. However, under some load conditions, the amount of bypass desired to avoid EGR fouling may lead to too much bypass, thus leading to intake manifold temperatures becoming too high for desired combustion characteristics or for system durability.
By appropriate modulation of EGR flow through a plurality of coolers and at least one bypass in a variety of modes, it is possible to both maintain cooler outlet temperatures above fouling limits, and also maintain intake manifold temperature at an appropriate level for combustion. The modes may be selected based on engine speed and load conditions, based on feedback including estimates of intake and EGR temperatures, and/or combinations thereof. Further, adjustment of flows within the various cooling modes may also be used based on operating conditions such as intake manifold temperature, and others.
Thus, rather than designing EGR cooling systems for the highest cooling load point (which may result in an oversized cooler for other operating conditions), the system can modulate the cooling capacity and partial bypass flow to use only part of that cooling capacity to target a higher intake temperature. The higher intake temperature can then lead to reduced CO and HC emissions.
Note that while different amounts of partial EGR cooler bypass operation may be used, the system may also operate between either full or no cooler bypass operation, with various modes in which different coolers utilize different settings.